1. Field of the Invention
The present invention relates to a circuit which monitors the RF output state of a radio-frequency circuit adapted for, for example, a microwave circuit.
2. Description of the Related Art
A conventional microwave circuit used in radar equipment has a monitor circuit set on the same circuit board as an radio-frequency (RF) amplifier for the purpose of monitoring the RF output state and fault detection of that RF amplifier (see, for example, “Rader Engineering Revised Edition”, page 166, edited by the Institute of Electronics, Information and Communication Engineers and published by Korona Publishing Company in Japan). The monitor circuit is configured to take a portion of the RF power from the RF output line of the amplifier using a directional coupler and obtain a DC voltage via an RF diode detector circuit. The DC voltage is monitored to detect variations in RF output power and faults of the amplifier.
The directional coupler is placed immediately after the output of the amplifier with space efficiency of the circuit board in mind. The length of the line which is coupled with the RF output line is normally selected to be λ/4 where λ is the wavelength of the transmit signal). Since the RF output end of the directional coupler is located on the amplifier side, in order to connect the output end to the input of the RF detector circuit, it is required to fold back the wiring pattern. This results in an increase in the size of the circuit board.